This investigator is engaged in a long term effort to define the neurophysiological mechanisms and anatomical interconnections of central neurons subserving cardiovascular reflex control. The present studies will focus on both afferent and efferent limbs of the vagal innervation of the heart. Initial studies of afferent projections will use horseradish peroxidase (HRP) techniques to determine the general locations of cardiac vagal afferent cell bodies in the nodose gnaglion and to define the regions of the nucleus tractus solitarius (NTS) in which their central projections terminate. In subsequent electrophysiological studies, single unit recordings from the cell bodies of cardiac vagal afferents will be obtained, and antidromic central stimulation will be used to determine whether specific fiber types terminate on different cell populations in NTS. In addition, the hypothesis of a presynaptic interaction between afferent inputs to NTS neurons will be tested by observing the effect of electrically stimulating other peripheral cardiovascular afferent nerves (e.g., aortic or carotid sinus) on the threshold for antidromic activation of the central endings of single vagal afferent neurons. Studies of efferent vagal mechanisms will evaluate the significance of the recent observation that vagal neurons controlling heart rate reside in two specific brain stem areas--the nucleus ambiguus (NA) and the dorsal motor nucleus (DMN). The hypothesis that these neurons selectively process afferent input from different receptor groups will be tested by recording from units in each area during manipulation of afferent input from selected cardiovascular receptors. Emphasis in these studies will be on the manner in which neurons in both nuclei process physiologic input from carotid sinus and left ventricular baroreceptors alone and in combination. Intracellular iontophoresis of HRP will be used to fill the dendritic trees of some of these cells to determine the extent of their anatomical connections with other brain stem regions involved in cardiovascular reflexes. These studies will provide new insight into central mechanisms influencing vagal control of heart rate in normal animals.